In a simplex two-way radio communication, an antenna must be coupled to a receive port in order to allow reception of a signal by a receiver, and similarly it must couple to a transmit port in order to allow transmission of a signal. For example, in a two-way portable radio unit an antenna switch switches the antenna between the receive port and the transmit port in response to control signals generated by pressing a push-to-talk (PTT) button. Additionally, in some portable two-way radios, an additional antenna port is provided to allow reception and transmission of signals through a remote antenna. The remote antenna may be an antenna on a car, or an accessory unit. Accordingly, a four port antenna switch is needed to provide for switching of signals through a standard antenna port or a remote antenna port.
Conventionally, the antenna switch may comprise one or more relays for switching the selected antenna (remote or standard) between the receive port or the transmit port. In this arrangement, when the PTT button is pressed, the relay is energized and switching action occurs. Additionally, a mechanical switch may be used for switching the standard antenna and the remote antenna to a selected port (transmit or receive) when an accessory unit is connected to the radio. Occasionally, the relay must withstand a substantially high power radio frequency (RF) signal from the transmit port, when the radio is transmitting RF signals. In some two-way portable radios the high power RF signal may be as high as 10 watts, therefore, a miniature high power relay is used. The miniature high power relays are expensive and also pose a reliability problem because the contacts of these type of relays may be eroded quickly due to conditions caused by switching high power RF signals. Accordingly, using mechanical switching means, such as relays and mechanical switches, however, presents a problem of reliability and cost.
Solid state switches provide an attractive alternative to the foregoing arrangement. FIG. 1 illustrates a common PIN diode switching network for antenna A. In the transmit mode the transmitter voltage V.sub.TX is coupled to the supply voltage while the receive voltage V.sub.RX is coupled to ground through inductor LRX. The transmit voltage V.sub.TX forward biases diode CR.sub.TX through inductor LTX and, thereby, couples the transmit port Tx to antenna port N which is coupled to ground via a resistor R. This configuration is very broadband since the transmit path includes only a forward biased PIN diode CR.sub.TX, while the diabled receive port R.sub.X isolation is provided by a reversed bias diode CR.sub.Rx. However, note that the diode CR.sub.RX can only be reversed biased by the maximum available supply voltage less the diode's bias voltage drop. The major drawback is that whenever the peak RF swings at the antenna port N exceed the available supply voltage, the RF signal will cause the disabled diode CR.sub.RX to turn on allowing power into the path intended to be disabled, thereby eliminating transmitter to receiver isolation. In land mobile communications the maximum supply voltage is 12 volts or less, while transmit power can approach 150 watts and transmit voltage swings can exceed 100 volts (&gt;&gt;12 volts). Clearly, the limited power handling capability of this configuration makes it unacceptable for land-mobile applications.
An alternative configuration is illustrated in FIG. 2. In the transmit mode, both PIN diodes CR.sub.TX and CR.sub.RX are forward biased through an inductor LTX by supply voltage V.sub.Tx to a conducting state. A quarter wavelength transmission line is employed to provide sufficient isolation between receive port Rx and the transmit mode. However, the major limitation of this configuration is that it contains a resonant element (i.e. the quarter wave line) that severely limits the bandwidth of the network and may cause transmit power loss.
One of ordinary skill in the art may appreciate that the arrangement of FIG. 2 may be duplicated in order to switch a standard antenna port and remote antenna port to one of the selected transmit or receive ports. However, this approach provides additional drawbacks. Because, one of the diodes must be reversed biased in order to isolate the remote antenna port from standard antenna port. Therefore, due to non-linear characteristic of diode, a reverse biased PIN diode in presence of a high power RF signal produces undesired frequency harmonics which adversely affect the performance of the radio. Filtering these undesired harmonics further reduces the antenna switch bandwidth and cause transmitter power loss.